Methodology:

Specimens were collected from each of 1 station in 2017 (B13) and 2 stations in 2018 (B16 and B17) during two consecutive cruises (RRS James Clark Ross: JR16006, 30th June to 8th August 2017; JR17007: 10th July to 5th August, 2018) following a transect along the 30°E meridian. At each station similar-sized specimens of the infaunal bivalves Astarte crenata and Bathyarca glacialis were obtained by Agassiz trawl. Surficial sediment < 10 cm deep was collected using SMBA box cores, sieved to remove macrofauna (500 µm mesh), and homogenised by stirring. Both infaunal bivalves and sediment were maintained under aerated seawater and returned to the Biodiversity and Ecosystem Futures Facility, University of Southampton at ambient temperature (1.5 + 1°C). Ten centimetres of homogenised sediment were transferred to transparent acrylic aquaria (internal dimensions, LWH: 20 x 20 x 34 cm) and overlain with ~8 L (20cm depth) surface seawater (salinity, ~34). Aquaria were maintained in the dark at ambient conditions (1-2 °C and ~400 ppm [CO2]), and randomly distributed between two insulated fibreglass seawater baths (LWH: 1.2 x 1.2 x 0.8m, Tanks Direct, UK). Three B. glacialis were introduced to each of twelve aquaria (n = 36; ambient 18, future 18) and six A.crenata were introduced to each of ten aquaria (n = 60; 30 ambient, 30 future). After a 30-day acclimation period to aquarium conditions, temperature and [CO2] were manually adjusted (1 °C and ~100 ppm CO2 week -1) to achieve near-future treatment conditions (3-5 °C and ~550 ppm [CO2]). We periodically measured pH, temperature and salinity and total alkalinity. The bivalves were fed ad libitum three time per week (100 ml cultured live Isochrysis sp., Tetraselmis sp., and Phaeodactylum sp.), and overlying water exchanged weekly. Experiments were run for 120 days (B. glacialis, 21//11/2017-20/03/2018) or 135 days (A. crenata, 08/10/2018-19/02/2019) after which animals were removed and fixed in 4 neutral buffered formaldehyde.

Oocyte images: Images of stained slides of histological sections from individual females were captured using a Nikon D5000 digital SLR camera mounted on an Olympus (BH-2) stereomicroscope and analysed using ImageJ v 1.48. Slides were photographed at X10 magnification. Replicate photographs were taken to ensure 100 unique oocytes were measured for each female. Composite images were generated for easy analysis wherever possible. Full descriptions and additional information provided in Reed et al. 2021 Frontiers in Marine Science in main manuscript and supplementary information.

Data collection:

Resolution:

Biometric measurements (i.e., shell length, shell height and tumidity (width) to two decimal places.

Wet weight to four decimal places.

Oocyte Equivalent Circular Diameter to two decimal places.

Instrumentation:

Mettler-Toledo InLab Expert Pro temperature-pH combination electrode

Mettler-Toledo InLab 737 IP67 temperature-conductivity combination electrode

Digital vernier calliper

Balance (+/-0.000g)

Rotary microtome

Nikon D5000 digital SLR camera

Olympus BHS (BH-2) stereomicroscope

Data quality:

Standard protocols were followed and data entry double checked by independent person.